How to tell when the serpentine belt tensioner is bad - Mercedes-Benz Forum
 
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#1 (permalink) Old 05-22-2010, 01:36 AM
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How to tell when the serpentine belt tensioner is bad

Hi everyone,

I was told that my belt tensioner is bad and needs replacement. My question is how would I know when the tensioner is bad?

My 1995 C280 has 130K, last time the belt was replaced was at 87K. I don't know if the tensioner was ever replaced.

The belt looks a little worn, no cracks just looks a little dry maybe. There a little of sound coming from the front. Some squeaking is audible but I don't know whether it is the belt or he tensioner or just that the pulleys need some WD40 cleaning.

So how do I know if the tensioner needs replacing? Any advice is highly appreciated.
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#2 (permalink) Old 05-22-2010, 07:13 AM
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Is the tensioner bouncing around?

Can you move it by hand? If it is frozen, it's bad.

Get a second opinion.

Do you trust the mechanic?
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#3 (permalink) Old 05-22-2010, 09:23 AM
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>>Some squeaking is audible but I don't know whether it is the belt or he tensioner or just that the pulleys need some WD40 cleaning.<<

You may need to change the pulley too. WD40 might help quiet things in the short run but it won't last and could break down what little lubrication is left which would hurt you in the long run.
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#4 (permalink) Old 05-22-2010, 02:02 PM
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M104 belt tensioners can be a pain. Sometimes there's nothing wrong with it.

One thing alot of shops (even the good ones) miss is the PIVOT BOLT. It's this giant 10mm hex-key bolt that goes through the center of the tensioner. THIS BOLT MUST BE LOOSENED 3-5 TURNS TO PROPERLY ADJUST TENSIONER.

I tensioned mine for more than 10 years without releasing the pivot bolt. It would still adjust, just hardly enough to replace/tighten the belt. Finally, my belt got to the point a month or 2 ago that it was so loose, no matter how much I adjusted the tensioning rod it wouldn't get tighter (or looser). So, tensioner is bad right?

Ordered replacement, disassembled entire front of car for replacement only to discover the pivot bolt. I never even knew it was there. I reassembled the whole thing (old tensioner still inside). Left the pivot bolt a little bit loose this time. Discovered I had stripped out a tiny section of my adjusting rod (because in the past I hadn't loosened pivot bolt). I pressed REALLY HARD to get the nut on the adjusting rod to go farter than the stripped point. Then, I discovered I could tension everything normally. My old, 1994 factory original tensioner was just fine, minus a squeeky pulley, a noisy shock, and a stripped tensioning rod. But since I had ordered all new stuff anyway, i decided to take it all back apart and replace it (electing to keep all the OLD parts as spares).

When removing the pivot bolt through the center of the tensioner I discovered it was extremely hard to turn. 10 years of tensioning the assembly without loosening this bolt had warped the bolt. In the process of SLOWLY getting it out, I broke it off in the hole. I had to spend 3-4 hours drilling it out. In the end I went up to a 1/2" bolt (due to the damage to the threads in the hole). I also had to use a 1/2" drill bit to carefully bore out the center of the new tensioner so the half inch bolt would fit. Reassembled without a problem.

So before you start thinking your tensioner is bad or a shop has told you so: check it out yourself. I know someone who paid $880 for this job on their C280. Not pretty. Loosen that pivot bolt and THEN try to adjust the tensioning rod. If there's already tension, then your tensioner is fine. You've got a pulley going out somewhere. 50% its the tensioner pulley. 49% the idler pullet (top passenger side of motor). 1% any other pulley.

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1998 C43 AMG 040-Black - Wife's
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#5 (permalink) Old 05-22-2010, 03:18 PM
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The recommendation came from an MB dealer when I took it in for an oil change special. Ofcourse the hope at these specials is to get you to do other sometimes unnecessary work.

The tech said that my tensioner shock bushing is bad. I can only assume he guessed that from the sound b/c he didn't take anything apart. I am looking to get my hands dirty sometime this weekend or next. But after reading the posts here I believe I can still go on for a while before anything really needs changing.

The tensioner doesn't bounce around and the belt is hard to impossible to move by hand. By the looks of it earlier the belt is getting some fissures on the edges but nothing terrible. Approx. how many miles do these belts last?

Thank you sulaco for your insight. It is very helpful to know. And Thank you all.
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#6 (permalink) Old 05-23-2010, 02:48 AM
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100 k is about a good time to renew - cheap enough
A swing on the torx nut will give you an idea if it's OK - an ordinary ring spanner about a foot long ideally needs an extension to move it easily

Also easy to renew the bearings to take all the guess work out of the noises - just press in - brgs about $20 - (the plastic idler on the 4 cyl cars has to be replaced - bearing cannot be pressed in/out)

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#7 (permalink) Old 05-23-2010, 12:24 PM
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Great pics.

You seem to have a lot of space between your fan and pulleys. I don't. I hardly have room to insert a tool.

Love my 1995 C280. I would gladly give up my girlfriend before I give up my Mercedes Benz.
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#8 (permalink) Old 05-24-2010, 10:07 AM
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When mine went the squeal just kept getting worse. You could feel the slack in the belt. I replaced the tensioner & pulley for about $90. There is a great DYI on this site that will walk you through it. It took me about 45 mins. In hind sight I would replace the schock while I have it apart. I think it was only 25-30 dollars for the shock. to give a little room to work I took the 4 screws out of the fan (removed it) then remove the whole fan shroud takes about 10 mins but gives you all the room you need to work. Mine was a 95 220 I see yours is the 280. not sure how much diff they are.
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#9 (permalink) Old 05-24-2010, 11:21 AM
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Quote:
Originally Posted by JH Easton View Post
Mine was a 95 220 I see yours is the 280. not sure how much diff they are.

about 8 less inches of room between engine and radiator.

seriously.

the m104 is a massive/long I6 shoe-horned into the engine bay. Even with the fan and shroud removed you have roughly 2-3 inches to insert tools and work.

In the end I pulled my radiator and a/c condensor to drill it out. You could get away with just pulling the radiator to give yourself about double the space. Might make better sense anyway, because you could VERY easily puncture the radiator with a tool.

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#10 (permalink) Old 05-24-2010, 01:08 PM
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Thank you. It's my first time messing with the pulleys and so I rather have more space than less.

On a different note, would you guys mind commenting on another post I wrote up. You all are more experienced DIYers than I.

http://www.benzworld.org/forums/w202...ar-fluids.html




Quote:
Originally Posted by sulaco View Post
about 8 less inches of room between engine and radiator.

seriously.

the m104 is a massive/long I6 shoe-horned into the engine bay. Even with the fan and shroud removed you have roughly 2-3 inches to insert tools and work.

In the end I pulled my radiator and a/c condensor to drill it out. You could get away with just pulling the radiator to give yourself about double the space. Might make better sense anyway, because you could VERY easily puncture the radiator with a tool.

Love my 1995 C280. I would gladly give up my girlfriend before I give up my Mercedes Benz.
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